Were Syd Barrett and John Lennon known to be friends?

Arguably no.

There is little to no mention of Barrett in any of the Lennon biographies that I've read over the years. That's not to say that they were enemies.

If anything I would characterize their relationship as that of "acquaintances".

Here is one horribly written observation that details a time when the two bands met:

"The Piper At The Gates Of Dawn’s tittle was taken for The Wind In The Willows and was recorded in Abbey Road at the same time as The Beatles were recording their masterpiece Sergent Pepper album. Both John Lennon and Paul McCartney would come in and listen to what Syd and his fellow cohorts we crafting and the influence of Pink Floyd’s album on The Beatles must be noted. They were riding high and were the kings of psychedelic London. Syd was hailed as a genius and lived the life of the rock star but things were about to take very bad turn and one of rock’s most famous stories was about to be written."

http://www.economicvoice.com/musical-memoirs-syd-barrett-part-one/5001258

Syd was always complaining that John Lennon owned a house while he only had a flat.

http://www.sydbarrett.net/subpages/articles/syd_barrett_story_by_syd_and_tho.htm

star_gazer: To put that quote into context, Syd was saying that to Peter Barnes. He's been Pink Floyd's music publisher since 1973, and he is who is the one quoted as saying Syd said that, probably at that time. Lennon is noted as having been a psychedelic hero of Syd's along with the band Cream - Syd even had a psychedelicized John Lennon poster in his flat, every bit a fan. A year later he was hanging out with him personally and professionally. I think at that time he probably felt the heat of competition within the Beatles' inner circle, directly from his hero.

Maybe Syd felt the new music publishing company could get himself and Pink Floyd more money when he said that to Peter Barnes? That would be the right person to complain to about it. (They had signed a poor record deal - a £5,000 advance over five years, low royalties and no free studio time.)

Who knows what conversations Syd had with John Lennon at EMI studios? This was all from the beginning year of Pink Floyd becoming a right studio band. One thing is obvious - once Pink Floyd's first came out and they were in the wave of psychedelia, appearing on Top Of The Pops - Lennon was already moving on to something else. So the relationship was more Syd looking up to John Lennon.