。
When choosing friends, keep a cool head.
Literal
Friends [object-を] choose time [topic-は] cool-[adverbial-に].
An elliptical advice-style sentence — the verb 'be' is implied. 冷静に ('calmly, cool-headedly') is the adverbial form of 冷静, formed from 冷 (cold) + 静 (quiet). The whole sentence has an aphoristic bumper-sticker feel: 'stay cool when picking friends.'