I would be hesitant to make a repair to it with all the liability these days. Most of the time I wouldn't do it, but if it was the right customer I might as long as they signed a waiver on the tag stating it's not a proper repair and they accept full responsibility for any water damage if the repair doesn't work.
With that said, if its a small puncture I think waterproof epoxy would be your best bet. Prep the area good by scuffing it with emory cloth and clean it with alcohol and then put a good layer on it. Some epoxy will melt some plastics with the reaction when its curing so if you're unsure test it first on another area of the tub where if it doesn't work it won't be a problem. Like a tab or something underneath where if it does melt thats not a part where the back side is open to the water.
Again, 98% of the time I wouldn't want to do it. What I might do is give the customer some ideas and let them fix it themself.