1° make sure you can prove what part of the software was there before the other founder joined google. This will put a clear limit to what google might claim as ownership. Dito for its contributions while he is hired by google.
2° inform (in a provable way) google immediately of the situation and ask them to clarify the issue with you ASAP.
3° check the laws in play by yourself. Don't rely solely on what google may tell you or on lawyer advises. It cost some effort, but as a future employer you better know the laws yourself on this topic.
As I'm in France the law is published on the web. It is thus easier to do. Don't know what the situation is in the US and your state. I developped an IT system during my weekends and I had to sort the IP issue with my employer. I not only managed to sort it out without paying anything more than one registerred letter, but I also managed to preserve a very good relation with my employer, which I value most. If both play fair and honnest, this comes out naturally. Beware that there are many traps you might be caught in. But don't be paranoid, there are traps for your employer too. It is really worth the effort to learn the IP laws.
If you clarify things with google, make sure you get a definitive conclusion that will be valid even if the google employed founder changes states or country, since law applyable to him may change.
Though, I am surprised because in America, many people at some point, have worked at two or even threee jobs during the week to make income.
So theoretically, if he were to get a consulting job with Microsoft during the weekends, will Google own a piece of Microsoft? That would be interesting to watch :)
The short answer seems to be yes though.
(IMHO could potentially be a useful tool for youtube. http://juwo.com)
I can see google making a case that he "used" code from google or something; remember it doesn't have to be true to take you to court or bog you down for months and thousands of dollars. For google lawyers it is signing few sheets of paper, for you it'll be putting your life on hold.
Best to resolve this before you start IMO.
It is like walking with an armed bomb in your pocket where you could have defused it by clarifying the issue with your employer. Beside you now have the advantage of the first move. Profit from it a wisely.
So before telling anything your employer, better learn how to do it, what you are required to tell your employer, and what you can keep for your self. There are things you can't undo.