RPGMemes

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.