Hi, I found this topic rather late, but here goes, with the benefit of reading everyone else's response.
I think first of all, welshwitch is right on, when she said a lot of time, people don't need advice as much as they need a kind and accepting friend who simply listens while they verbalize their thoughts and organize their feelings. With Ennis, this may be difficult, so a few words of advice may be needed.
Secondly, I think the advice depends on the time and situation in the movie. For example, if the call came just after their separation in '63, I'd say send the letter or postcard right the way, just to stay in touch, and maybe arrange to get together. They should have said more at the truck, but given the circumstance, staying in touch was the least they could do. How they proceed from that point would depend on them. Especially with Ennis being engaged to Alma, he'd have to do some difficult thinking and making a hard choice.
If the call had come at the reunion, with their own marriages still fresh...it would be difficult, especially with young children involved. I think just listening and being supportive is all a friend can do.
If the call had come after the divorce, I think it would be up to Jack, if he wants to take the reins, and offer to take Ennis up to his folks' for a visit. No commitment, just a look-see. And Ennis would be wise to at least explore the feel of the land. Once he met Jack's Ma, I am sure they'd be a couple building a cabin. There'd still be the old man to deal with, but Ennis is tougher than he looks.
If the call had come at the river scene, I'd say move to Texas. Start fresh, see what happens. It can't be any worse than what he had, and he'd be closer to Jack.
If the call had come at the final meeting, I'd knock some sense into them and make them stay another two days up in Lightning Flat, make Ennis quit his job, and Jack divorce Lureen, and the two of them build that cabin. By that time, they'd already committed to each other to the point of no return, and the children have grown. Twenty years!
But, hey, that's just me. I am a nosy-busy-body who tend to stick my neck into other people's business, and ended up getting chopped in the process.

No, seriously, I am just so sad to see them breaking up that way, I'd do anything to have them kiss and makeup.
Alternate universe or not, Ennis and Jack forever.
