Leonard, that was the first song we played on my dads 50th! but then we did have a beatles tribute band. I would say any motown compilations (diana ross etc), things like Jackson 5 and Wham are good floor fillers.
Also, little suggestion that went down a treat at my dads party - get a nice book/large diary and leave it on a table somewhere and ask everyone at some point in the night to write a message or comment. It'll be nice for your mum to keep and look back on - have a great night!