I recently replaced the cassette and chain on my wifes bike only to discover that the middle chainring was worn out too. She did one ride with just the inner/outer chainring available before I bought a replacement. I bought and fitted a 32T Deore chainring (she has a Truvative chainset) but the chain is still slipping on the middle ring.

Any ideas what may be causing this? Could the chain have been damaged by attempting to use the middle chainring suring one 2 hour ride?

Quite possibly, had exactly the same happen on a friend’s bike. If the old ring was badly worn it wouldn’t spread the load around the ring, only on the first tooth engaged whilst the chain on the others was slack, putting a fair amount of stress and wear on that link. You’ve then got ,effectively, a worn chain trying to work with a new ring.
We swapped the chain and cassette for new items and it worked fine, put them back on and it slipped.

“Could the chain have been damaged by attempting to use the middle chainring suring one 2 hour ride”

Its more likely to either be:
-You’ve not set the chain to be the correct length for the casette/chainrings [round the biggest chainring, round the biggest rear sprocket (NOT through the mech) and then overlap of one outer/one inner plate]
-Just too new, the chain hasn’t worn to the chainring yet (whenever I replace the chain/casette/rings, it takes a ride or two to stop jumping/skipping and so on

It could be something else
And it could also be that the old chainring was so worn, its worn the chain too.
Use/buy/borrow a chain wear tool and see how stretched it is. If the old ring was really bad and has worn out the new chain, it’ll be obvious. Get a new chain (though could’ve worn the casette too). If the chain tool is miles off fitting the chain and it has no play in it whatsoever (and no or very little obvious wear on the little inner rollers (the bit that goes in contact with the rings) then its probably better to make the new ring ‘fit’ the partially used ring

I checked the chain length and chain wear and both were fine. I brought the bike out again today and the problem seems to have sorted itself out. I reckon it was either a stiff link in the chain (it was lying in the garage out of the box for about 2 months after 1 use before I fitted it) or as james says it just needed to bed in, although I’ve never experienced this before. Anyhow, seems to be on the mend, thanks for the replies!