Regardless of the age, gender, marital status, life experience or streetwiseness of the @Moocherbot's mum, it was very odd for the workmen to send her out to buy the silicon, leaving them u supervised in her home. Surely this is the central issue?
I firmly believe this needs to be reported to Carpetright and the police - and if the OP's mum does not want to do this, I think the OP should still report it - not to 'infantilise' her mum, but to try to stop these workmen doing this to someone else.
I can think of no legitimate reason why workmen would do this - the only reasons I can think of are dodgy at best - casing the house for valuables, for a letter robbery, stealing stuff there and then, looking for keys so they can come back for some nefarious purpose.
Maybe they didn't find anything worth stealing at the OP's mum's house - I assume she has checked carefully to make sure there are no valuables or keys missing - but I would still report it and change the locks.
Does that infantilise the OP's mum? Maybe it does. But if she won't report it, then the OP must, in case another person gets robbed or worse by these men.
Doing nothing is not an option, surely?