I’m assuming that neither your wife nor her mother ever converted to Judaism? If either one did, then you have no issue. If neither did, then… well, maybe.
According to Reform Judaism, the baby is Jewish when one parent is Jewish and the child is raised Jewish. So, if the mohel is Reform, you have no problem.
However, according to Conservative and Orthodox Judaism, Jewishness descends from the mother’s side. So, if your wife’s mother was not Jewish by birth (and didn’t covert), then your wife would not be Jewish (unless she converted) and neither would her child. I do not know how the mohel might react if that’s the situation, and if the mohel is conservative or orthodox.
The orthodox/conservative process of converting a baby is fairly simple, however, and usually involves taking the child to the mikvah (ritual bath) and appearing before a panel of rabbis to swear that the baby will be raised Jewish. It’s helpful if one of the rabbis knows you, of course. I assume that, if the mohel refuses to do the circumcision on grounds that the child isn’t Jewish (by birth), he/she might be convinced if you promise to take the child through the conversion process. However, I’m just guessing here.
Let me also say that, with many Russian immigrants, there’s an understood “don’t ask, don’t tell” side to things. All religions were discouraged under the Communist regime, but Judaism especially received harsh treatment. Consequently, there’s lots of uncertainty. People who think they were Jewish might not have been, and people who think they weren’t, might have been. Therefore, there’s now a fair amount of deliberate closing of the eyes. So, one way out: if your wife was raised Jewish and thinks of herself as Jewish, just don’t bring up the issue of her mother.
And, mazel tov on your child, and all the best for you and your new family for the coming year.