How can I make my companion focus on healing another player?

As mentioned in other answers there does seem to be a priority system in place but I don't know of anyway to change that.

However a possible way to help in your situation is to set your friend as your focus target and put your companions heal ability in a keybound quick slot.

You set a focus target by selecting another character (friend, enemy or npc) and pressing ALT-F (the default binding). You will see you have a new player icon and health bar on your screen. Now pressing End (again, the default binding) will cause your next ability to target your focus target.

I haven't tested but pressing End and then your companions heal would cast it on your target.

There is no way to make your companion focus healing on a particular target. All you can do is to disable all offensive abilities and turn on companion's healing stance (can't remember the exact name now).

From what I have observed, healing priority is decided by current health level and if it's the comapnion itself or a player (all players in group seem to have more or less the same priority, but self heal is higher in prio, which is kinda annoying when my Mako is at 60% hp, me at 20 and she heals herself)