Do as requested, or get all "gung ho" and try to find another buyer in January in a falling housing market. (Two years ago I'd be telling them to "do one" and have it back on the market for 5k more)
Unfortunately a builder could put in a beam the size of the Forth Bridge but (certainly for the current buyer) it will need SE input. They would be unwise and unlikely to ignore the professional advice they have paid for to accept "my mate's a builder, he's put another one in"
I think (assuming you find a reasonably priced engineer, and he doesn't find a simple solution such as additional propping off a spine wall etc), your bigger issue will be finding a builder who will hoike a £100 lump of timber up there without charging you thousands - someone who has a tower scaffold or can see a route through a window and hole in the ceiling.
If the sale falls through and you get your builder to do a like for like (or a bit bigger - there must be a reason the original failed!), the new problem you'll face is trying to make it look like it's been there for 20 years and not two weeks - because that will set the alarm bells ringing for the new surveyor -
As an aside, how can people have so little insulation in their loft theses days!