Hi. I am currently working in US on a H1b visa as a doctor. My
girlfriend is a german citizen and has come to US on a visa waiver
program. We getting married next year. Now till that time she can only
stay in US for 90 days. We are planning a holiday to Canada soon. Now
if that is during the end of her stay in US, while we re enter US
would she get an extension to stay for another 90 days or not?

nope she would not. The problem with the waiver program is that there
is absolutely no extension of the visa. Entering Mexico or Canada and
then re-entering the states will not give her an extension or another
visa. I know because I've been in this situation before. Apparently
there is some exploitable workaround but I never bothered to find out
since I don't really like the states that much anyway. The milk never
goes sour, the vegetables never go off and everyone gets cancer when
they are 30, make the connection.

Thanks Gorilla