@roger,
In all seriousness, this guy Bill needs to be kept on the radar at all times.
It's not enough to just say "it's her life" Of course it is. But, if she's being blind to obvious things such as knowing a friend said he's a compulsive liar, she's also being blind to being taken advantage of. What if (and I'm saying this not particularly about this situation), this con man takes her for everything she's got? She's only 60. She could be living for the next 30 or more years flat broke, with everyone saying "if only we'd done something" He could also cost the mother her current job, and at 60, that is old enough to make it very difficult to get other employment.
There is no one size fits all solution, or one that's guaranteed to work, or at least work on it's own.
Found Soul said it best saying be looking for physical evidence of his deeds. That might persuade mom, it might not. Worth a try.
Not knowing this guy, can't say what would work on him. Could be anything from a strong arm technique (if he's a wimp or coward) to something more sophisticated if he's really in it for the long haul.
If his interest in mom's money isn't that strong on a 1-10 scale (i.e. she doesn't have that much) telling him that you've got his number and he'd better ******* watch his back might be enough, maybe with a little push to his back to let him know you're really watching it, capeesh?
If that doesn't shake him because he knows he's got her hooks in her good, and will stand by him rather then her own flesh and blood, you have to make it more trouble than it's worth for him to stay with her.
If that means telling mom you're head over heels for him too, and insist he participate in every family activity, so be it.
If mom is picking him and a girlfriend up to take them to his house, I'd tag along, and when they get to the house, get out with them and invite yourself into the house, settling down and not budging, being really dense to all suggestions he leave. Basically, making myself an ever present fixture in his life, keeping him off balance between friendship and veiled threats.
Con the con man.