Okay so last week I played a game with a Teen Theme and my opponent tells me I can't use Speedballs 18 defense to boost Stargirl V's Defense and I was very confused by this since Speedball (Spiderman Ally) is using his wildcard to delcare JSA.

He tells me she can benefit him but not the other way around which just seems silly. For all intents and purposes Speedball is a JSA member for the turn and the power works both ways. I mean what's the point of the TA if I can use it both ways?

Is this true? Can wildcard only be benefitted but not bestow their defense value in this situation?

It all comes down to "using" a TA vs "Possessing" a TA. In the case of Stargirl, she possesses the TA so she can lend out her DV to anyone who can use. In the case of speedball, as a wildcard he can use friendly TA's, but he does not possess them. So, only a figure that possesses the JSA TA can lend out DV's per the player's guide description of the Team Ability. Your opponent was correct.

It's true. Here is the current wording of the JSA TA from the Restricted version of the Player's Guide:
The key words here are 'use' and 'possess' (hence my emphasis). A wild card does not possess the TA, they merely use it, so a wildcard can receive a DV (since that only requires use of the TA) but cannot lend it (that requires actual possession).

And it is the same wording for ALL the "sharing" TAs -- Sinister Syndicate, Batman Enemy, Defenders, JSA.

Fantastic Four also makes a difference between using and possessing - healing will only occur when an actual Fantastic Four member is KOed, but wildcards get to heal as well when that happens.

And if we are going for a complete answer, we may as well mention that Thunderbolts are considered to possess the TA they choose (so T-bolting LE Antman to Defenders or JSA is still valid) while Mercenaries merely use the TA (and thus get the wildcard treatment).