It's funny, because I very nearly had this problem recently. I hadn't spotted it, but the condom had ridden right up to the top whilst she was on top (this is why many WGs will check that they can still feel the bottom of the condom when you are merrily fucking away). She blamed it on a new batch of condoms from the clinic, so I wonder if this is widespread (this was in the West Midlands). Was very flattered when she said that we would have to swap it for an extra-large one, but didn't fool myself that I had turned into John Holmes overnight. (I guess that reference shows my age - for the younger readers he was the male pornstar of the 70s and 80s..).
Perhaps she'll have learnt the hard way that in future she needs to make this occasional check that most WGs seem to do to see that the condom hasn't ridden up. Ultimately it is her responsibility and I can't see how she can blame you.