Although it looks like her, this lady is not 'Mantha. We initially think it is, who else would Lynne be rollerskating with, but when this lady mentions Merv we have to assume it isn't 'Mantha.
Let's be honest, Lynne walked straight into this one. The lady has deliberately directed the rollerskating outing to go past the theatre in which Mervyn The Memory Man is performing. Either that or the sight of the theatre made the lady remember about Merv forgetting her birthday, which in a way is ironic as he is a memory man. This is not a big town, we know that from previous stories, so it would seem that Lynne should know that her friend's husband (I think we can assume that) is a memory man and that he is known around town for having an amazing memory.
Maybe though, he's not that well known and this is his first attempt at a cabaret show, or revue. Let's look at the evidence: Lynne's never heard of him, the lady has forgotten where the show is and there are only two people outside, and they don't look that interested. It would appear more likely that Mervyn has forgotten his wife's birthday due to stress of having invested a lot of time and money into his show and it is being deemed a critical and commercial failure. Maybe his wife should think about that.