As a DM, ordinarily there are no rules for using fellow PCs as improvised weapons or for attacking incorporeal creatures with magic items other than weapons. However, there are a few things to consider with this approach:

• If a player is reduced to doing this, they likely do not have any other magic items at hand that would make suitable improvised weapons (An aside - how do you have a party member with magic armor and another with no magic weapons or implements that might be used as weapons? Maybe follow this combat up with a discreet review of the PCs magical equipment and equalize if there's a large discrepancy)
• it's funny as hell

Based on these two facts, assuming 5e rules, I'd let it work as a two-handed weapon attack, but the attack is made with disadvantage because the plated halfling with magic armor was not properly balanced for combat, and if it connects, it deals 1d4+STR mod damage to the ghost, and half of that to the halfling.

Step 1 : ask a player if they have any ideas Step 2 : do a google search for about 30 seconds to see if you can find something easily Step 3 : Rule with your guts, take a note of it to check between session, and MOST IMPORTANT STEP say this : I’m going to rule this like that for now and I’ll check in between sessions for a correct ruling. Do not use this here today as a final conclusion in a later session please.

This varies hugely from DM to DM.

When you're at the table, the most important thing is to keep the game running. Keeping the game moving and everyone playing is more important than "getting the rule right" - so there's a number of ways to do this - and different tables have different feelings, but here are some options.

1. If your table cares heavily about "getting the rules correct" you can look it up. Some players place a lot of value on "getting the rules right", they want to know for certain what the rules of the game they're playing are, and it's important to them to have a consistent ruleset to play within. For tables with this mindset it can be worth taking a moment to look up the rule, so long as people are happy with the flow of play being interrupted. This is an exception to the general "Keep the game running" priority.

1a) 5th edition doesn't have hard and fast defined rules for *everything" (in this case you could use "improvised weapon" rules and treat the halfling as an improvised club)

1b) A lot of DMs are much more happy to "let something slide" if the player isn't gaining any significant mechanical advantage from doing it... So interesting note: in 5e, ghosts are only resistant to damage from non-magical weapons, not immune, so "swinging an improvised halfling" is less good as an attack than just using your weapons. Lower expected damage, lower chance to hit, might damage the halfling. Since the player isn't even getting an advantage from doing this, many DMs would be happy to just let it happen.

Assuming you want to keep the game going, here are some other options.

2. If the game doesn't have a rule, you can use a rule you know from another game. 5e often doesn't have strictly defined rules for niche situations, but 4e and 3e often did have rules for those things. I often use an older rule I remember if 5e doesn't have a rule for it.

3. Go with your gut. Just decide what you think should happen given your understanding of the game world and metaphysics, then go with that.

4. If your table likes it, use the "rule of cool" and just allow anything that is funny or interesting or cool.

5. If you're convinced a rule exists, but you can't remember it, a really good practice that I recommend is as follows:
   a) In the moment, let the player do the thing in the fashion that's most favourable to them.
   b) Let the table know, this is a one-time ruling for the current session, not a house rule that's permanently in play
   c) Make a note of the situation in your DM notes
   d) After the session, do the research to look up the rule, and take the time to figure out how you think it should work. (If you have mechanically minded players, they might be interested in helping with this kind of thing.)
   e) Let the players know what the rule will be going forward. \

Fundamentally, what's most important here is NOT what method you use, but that your players know how you run the game and are happy with it. Any approach can work if the table likes that approach, and different approaches work better for different groups. There's no "right" or "wrong" answer other than "what works best for your group."

Google it to see if someone else has a reasonable answer. If not then try to come up with one yourself based off of other rules in the game.

Best way is to do what the meme says. Make a snap judgement and roll with it. If a player objects, look it up post-game and make a modification. Don't retcon what happened, but give a bonus/penalty to a player in order to keep things in line. Or create a scenario in which the ruling would be correct and give the players a hint to discover why.