What are the tax consequences for non us mother to transfer real property to U.S. citizen son?

[curiousvi]

Here is a possible situation..

A son who is US citizen is considering gifting real estate property to his mother who is permanent resident (green card holder)
He is planning to quit claim deed to her and I believe he or her will not have to pay any excises tax as it will be considered gift? I believe he can do as above, please confirm?

Now if by chance his mother decides to give up her permanent residency or forced by agent at airport to give up because she might not come back within 6 months or due to any reason residency is taken away from here...(so now she is not a US person) ....what will happen to property that now she owns (if son transfers as above)

I believe she can still remain owner but after loosing permanent residency (green card) and can still quit claim back to her son in future but what about tax consequences including income/gift/estate taxes to both ...mother and son?

To my understanding non us person can gift US situs real property but exemption is only $60k so if real property we are discussing is above 60k than mother (who is not non us person) has to pay up to 40% in estate tax? Please explain....