Took my friends AK out today and had a Mag that was causing it to do double taps. All other mags were working fine but that one mag would regularly do a double shot.

Can a bad mag cause that, if so how?

Should he send the firearm to a gunsmith or just toss the mag in the trash and call it good enough?

I'm not and AK guy so any advice is appreciated. Thanks!

No a magazine should not cause this! I think its either it has a sear engagement issue or messed up disconnect on the hammer. What kind of trigger is in it. I have seen a lot of bubbaed trigger jobs do this.

No clue what trigger. It's Romanian, looks pretty standard and mass produced, wood butt stock, wood fore grip, plastic pistol grip. He got it for a few hundred bucks from a friend so who knows that has been done to it.

I would liked to have experimented with it more but we were at a public range and didn't want to draw to much attention. Just weird that it would do it with one mag and not the other.

Thinking it might be best to just buy a new internal parts kit and replace the springs, hammer, trigger, etc...

No, not a mag issue.  Swap out the FCG and retest it.