I see little point in burdening your friend over your feelings that will not be reciprocated. It's fair enough to develop feeling for someone who has been there for you, but as you admit, it won't go anywhere. So acknowledge them to yourself and if you feel you are ready for a new relationship go look elsewhere.
Maybe, as you suspect, her girlfriend has detected your interest and is claiming your friend, as it were, as her own. I'm unsure the reason for it is important though. If you socialise with them occasionally ensure it stays friendly and try not to concentrate so much on your friend that her girlfriend feels you are competing. And ensure you socialise with others too as then I suspect you have a better chance of meeting someone suitable. Not so easy if you are playing gooseberry to another couple.