At times, Thanos and Warlock fight or have small disagreements (like those times when they have killed each other) and sometimes they team up. At one point, Adam says he trusts Thanos most to protect the reality gem. So are they friends?
At least when Starlin wrote the characters, it does indeed seem that Thanos and Adam Warlock considered each other to be friends. As far as I know other writers, such as Hickman, continued to portray Thanos as evil.
I found this image here. I believe the panel is from the end of Infinity Revelation.
In short, no. In long, they are both. Frenemies or "The enemy of my enemy is my friend" by Sun Tzu explains their relationship quite well. It has been a complicated relationship between the two, over the years. Adam is Life's Champion and Thanos is the Death's champion (Vol. 1, No. 11, 1976, Comic 02153 - Warlock - The Strange Death of Adam Warlock) .
They team up to face mutual foes and objectives but they don't seek each other out for drinking at the local tavern, if that's what you mean.
In their 1st meeting, Thanos tricked Adam (Life's Champion) into defeating the Magus (Life's Champion) by asking for his aid. Magus was Adam of the future, except he became evil and traveled to the past. Once the Magus was defeated, Thanos (Death's champion) was free to commit total stellar genocide (using the soul gems) and would become Adam's enemy. Adam would go on to die (not really, soul world in the soul gem), come back and defeat Thanos and then return to his death. That's the basics of it.
Then Mistress Death brought Thanos back to life. Thanos collected (again) the soul gems (now called infinity gems) and used them in a gauntlet form to become God of the universe and snapped half the universe to death. Adam (from inside the soul gem) returned to life to thwart Thanos. After thwarting, he teamed up WITH Thanos to get the Infinity Gauntlet from Nebula. (once again, the enemy of my enemy is my friend)
Frenemies. Yup. I could go deeper but that pretty much summarizes their relationship.