Do not use mastic .I do not know how to fix the leakut yes ust the thinset.

Depending on how big the leak is. I use the spray on undercoating for cars and spray the inside of mine and that stops most leaks. There is also a repair product at Lowes that is like an expoxy and it seals, even underwater. She can also pur a good coat of thinset all over the inside and then seal with the spray on undercoating. I use this stuff a lot for birdbaths and inside pots to waterproof. It can definitely be fixed. I can get the name of the product at Lowes but it will be in the morning as it is in my studio and it is dark and rainy here. Hope this helps. As everyone has told you, adhere the tess with thinset.

thanks so much for the help, i will pass the info on so she can get it sealed before she decides if she is going to mosaic it