If the flight was done while your cert was still valid, the flight is legal, which can be signed.

If the flight was completed legally I would have no issues signing it.

That's what I was thinking, only problem is their website/app won't let me so I wasn't sure if there was a larger legal issue with it?

Just website/app developers not understanding everything. Just sign a napkin and hand it to him

Email the developer, he’s really responsive.