Can I exhaust teammate's unit to pay additional cost for Meditation?

No, you cannot exhaust a teammate's unit to pay for the additional cost of Meditation.

According to RiftJudge FAQ #7727, a player can only spend buffs or perform cost-related interactions (like exhausting) on units they control, unless a card explicitly specifies otherwise. Since Meditation requires you to exhaust a "friendly unit," and "friendly" in Riftbound refers to units controlled by you (or your teammate in 2v2 scenarios, but the restriction on cost payment remains), the general rule for paying costs remains that you must control the resource you are utilizing.

Furthermore, Rule 702.2.b.2 and the principles established in the FAQs confirm that you cannot perform cost-related actions on units controlled by your teammate.