Return of Reckoning

All ironbreaker buffs that can affect the Oath Friend do so only if it is in a range of 50ft. Is there a way (addon or something else) to know at which range the Oath Friend currently is if this ally is not guarded by me ? The last statement is important because in RvR people are not always in my group, hence I can't guard them and see through Teather the range between them and me.

Check the Wiki-page "Addons" https://wiki.returnofreckoning.com/Addons
You'll find Guard range there which is already helpful if your oathfriend is your guarded target. Maybe something new in there for oathfriend.

You have Tether which connect you with a line on screen with guarded person [usually person that has oath on it] also guard range in Enemy.

Havent heard abt separate addon / indicator for oath.

Go to https://tools.idrinth.de/addons/guardpack.
Part of this package is the addon called Teather (with that particular spelling).
When you guard someone - it draws a line from you to the guardee. If they are in guad range (0-30 ft) the line is green. If they are out of range but relatively close (30-50 ft) the line goes blue. If they are farther than 50 ft - the line goes red. That will help you with your problem as long as you oathfriend someone in the same party as you. Beyond that - i am afraid there's no way to tell if you are within range or not

Thank you for your answers. I updated my question to specify that I wish to find a way to know the distance for an unguarded ally. I already use an addon which can show the distance of my guarded.