Well, except threaten the friend with lethal bodily harm. That might interfere with their friendship, although it would solve the ally/enemy dichotomy.
It leads to splendid inter-party dynamics, I'm sure.
Wizard: "Oh no! The Orc Paladin of Warhuggles has marked me! If I attack anyone else but him I'll take a penalty on my spell attack rolls due to split concentration, and my poor squishy buttoks will be zapped!
Fighter: No worries, chum! I'll just mark you instead, and you need not fear his creepy glowy burnination.
Wizard: Great! So how does that work?
Fighter: Oh, it's easy. First I'll hit you with my greataxe. Try not to flinch too much if you don't want to lose any important fleshy bits.
Wizard: But I like my fleshy bits!
Fighter: Oh quit being such a baby. We can always sow you back together after the fight. Now close your eyes!
Wizard: *eyes closed* I don't know about this...it's just one hit right?
Fighter: Yup.
Wizard: And then no more the with hitting, and burning, and robes becoming soaked through a mysterious process I am completely not responsible for?
Fighter: Yu-wait. Did you just wid--
Wizard: Shut-up!
Fighter: Well here we go...
*Kasplatter!*
Fighter: Uh...you ok?
Wizard:.....
Fighter: Oh come on...just a flesh wound, right?
Wizard: Fighter...
Fighter: Yeah?
Wizard: Did you just...crit me? With your friggen Greataxe of gory critting?
Fighter: Uh...maybe....Hey! who wants Ice Cream?
Wizard:....I hate you.
Fighter: It's not that bad. Look, the glowy mark is gone now!
Wizard: Wheee...so is that it? What happens now?
Fighter: Now it's simple. If you try to attack anyone but me, I'll gut you like a pig. Problem solved.
Wizard: So basically...we just kill each other now?
Fighter: *Chuckle* Don't be silly.
Wizard: Well that's a relief...
Fighter: I'll kill you way before you take me down
Wizard: *facepalm*