All seems like good advice, I agree it sounds a bit dodgy to me, if he owed it of course they could prove it no one is going to pay without proof. I wouldn't use a solicitor yet, as above right back asking for proof and give them a month to prove it, they will if it's genuine. See what happens and then maybe appoint a solicitor although they are expensive for what they do, so avoid them if you can.
Depending on how big the estate is can't you make a provisional payment to the beneficiaries explaining why you have to retain £60k or so for the time being?