Friend has a small leak in the cooler, in front of engine on the top... prestone mixes in water and goes out the exhaust...
is this fixable by welding those by a professionnal? Or needs a new cooler?

I think you are "allowed" to braise up to 3 tubes closed and still have it perform adequately.

^^^^ tig braise with silicone bronze filler wire

does not ring a bell to me but will share with a welder i guess he will know!
Thanks!

Buy a new one and be done with it. More leaks to probably follow if you fix it.

Difficult to repair, especially if in a salt environment. The repair would need to be the same method as the original, soldered vs welded. If it was soldered the Silicon bronze melts at a higher temp than solder therefore would end up chasing the crack all the way around. If its welded then silicon bronze would be relatively easy.
I would grind the crack with a very small burr, walnut shell blast it, 5 min epoxy and run it until the new one I ordered prior to doing any work arrived.
Diamond Performance Parts have treated me very well in the past. Or if new is not in the budget, reach out to Sick Stinger

Great input guys! Thanks for sharing!

Had the same problem with 1 tube on mine. Coolant system pressure test confirmed it. I was advised to not braise the tube as it "could" potentially cause others to leak. Advised to use a product called Allmetal to fill each end of the leaking tube. Holes are small so I used a coat hanger end to push about 1/4" of compound into each tube end. Once hardened, psi test ok. Going strong on 3 maybe 4 years now.

As stated earlier, Merc states 3 tubes can be braised shut before HE capability is reduced.

Thats easy to try! Thanks!