There are valid reasons to be angry with the Lib Dems and vow never to vote for them again, but vowing to not vote for them again over AV is illogical. They have always wanted PR, they got a vote on AV as a conpromise which is the closest anyone ever got to achieving it, and there's no other party that is pro-PR that has any chance of getting power under the current system (except maybe some regional parties and they're actually working against their own interests by supporting PR). The current system also makes it a requirement to vote tactically so ruling out voting for any party is an extreme position.
On the other hand, the current system also means many votes are wasted and maybe your friends know thier vote wasn't going to matter anyway, bit they prpbably sgouldnt encourage others who moght be on different circumsrances.