Their friendship was a source of strength for both of them.
Meaning: This sentence describes how the friendship between two people made them both stronger.
Choice of Words: "Source" means a place, person, or thing from which something comes or can be obtained.
Alternative Expressions
Their friendship was like a rock for both of them.
Meaning: This means that the friendship was very strong and reliable, just like a rock.
Choice of Words:Rock is something that is strong and stable. Like is used to compare two things. When you put them together, "like a rock" means that the friendship was very strong and reliable.
Their friendship was a warm blanket for both of them.
Meaning: This means that the friendship made the two people feel comfortable and happy, just like a warm blanket.
Choice of Words:Warm means something that makes you feel comfortable and happy. Blanket is something that you wrap around yourself to keep warm. When you put them together, "warm blanket" means that the friendship made the two people feel comfortable and happy.