Don't tear into the guy before you. Just say, "I'm sure this was built with an initial objective in mind. And it served the purpose well. The firm is doing well and expanding. I would like to know the best approach to make this product grow with it."
Then, ask the firm to define the objectives/requirements again now that you've found a *few* issues.
1.) Do you want the project to be re-built for _stability_?
2.) Do you want me to improve the _time to market_?
3.) Would you like me to leave it as is?, but be aware that extra QA will be needed.