Of course you should feel guilty, you gave bad advice, but its not as if you said invest in ABC ltd and it panned and now they have nothing.
Investing in property is always , always , always a good idea LONG TERM. So if they intended to keep it for st least 10+ years or so, then it was silly telling them not to do it. People get burnt when they can't leave the money for years so it flattens out the dips and rises and carry on upwards like property has for the past 60 years in a finite island with a ever rising population.
As they age and may need nursing care, nursing home etc, I suggest they consult an independent financial advisor , ageing people shouldn't be leaving money in the bank, not yet anyway, if we get double digit intrest rates, then hell yeah, but not now even!!
Focus on the money is still there and if you want to make amends READ, there's lot of excellent investment advice out there, a good study on Martin Lewis website will equip you with great advice even, so you're better informed for your own financial health. If you're parents aren't financially savvy, chances are you're not either and that something that needs fixing (as both of you demonstrate).
Sorry to be harsh, but I'd be less concerned about guilt, and more about learning from my mistake.