I do not see any issue here.
Your parents were in TPS status once it was approved and as long as they maintained it, there is nothing to worry about. If issue arise at PoE during next travel on B2, you could easily prove that you were in TPS status (with approved letter from USCIS) and left the country before it's expiration (return tickets and also immigration entry at Nepal which is stamped in passport)

I know Passport doesn't have any stamped for exit from USA. But passport will also NOT have stamped even if they left the country in B2 before 6 months expiration.

To answer daverazz question about F1. If you violate your F1 status while you were in TPS, then at the end of TPS, you are not automatically reverted back to F1 status. You have to apply for reinstatement  or leave the country and come back in F1.  But you didn't mention the cause of violation of F1 status. I don't think your logic here between B2 and F1 works. Only way you can violate F1 is either you didn't take full credit hours or worked outside using TPS EAD (which is still up for debate and I have not found convincing answers).

Hope this helps.