A weld a braze a solder blob are all castings.
Metal melted onto a place and allowed to solidify.
The only difference between brazing & a casting is the absence of a mold.
SO the ultimate strength of the joint comes down to the strength of the alloy.
A brazed joint is always a lap type of joint between two very close fitting parts and the strength of the joint is determined by the amount of overlap.
So you have a forged frame lug with a machined hole into which goes 1/2 ( or more ) of tube.
A highly reducing flux is melted into the joint (usually borax based ) then metal is melted in and flows under capillary action to fill the void.
There is a tiny amount of penetration of the filler rod into the base metal but the phases formed are quite brittle and the braise metal is also quite brittle.
Some high alloy steels can not be welded because the heat causes phase changes or grain growth in thin sections.
Nearly all braisings fail through the filler metal.
I do not know what alloy your mates frame is made out of nor the alloy used to weld it but fillet brazing, some times called bronze welding is not particularly strong and has a very low fatigue strength.
The deposits themselves look nice, very neat & uniform but have been done too cold and in too big a step.
If a student presented that to me he would be failed.
" Bronze is never a good idea on anything that vibrates and it is really more of a glue than a hole filler"
Trevor, I am curious about your statement above,, (I am not arguing just asking for clarification)
As I have seen a LOT of gas and oil tanks with brazing on them, and in many cases many oil pipes and fuel lines seem to have been sweated together using brazing as well. I have a vintage Triumph frame that was in an unfortunate fire many years ago sitting here as well, and the owner wants to restore it. I did some testing and the metal is not excessively cooked but we plan to install the frame in a jig, check for straightness and re-braze all the joints and casting lugs.
I have some old printed articles here and they state that the factory for BSA & Triumph used a hearth system to build the frames and the castings were heated up in heath furnace and then the plain steel frame tubes were inserted with flux and brazing. This is clearly visible on many frames when stripped. I follow some metal fab and industrial engineering forums and tech (always trying to learn) and much discussion on such over the years... Some speciality builders are using Brazing as a signature artform method as well... for example check out this beauty.
(See Pix).
If I am off the mark here, feel free to correct me.
(Known as "Weld-nrop" some fetishists find such craftsmanship stimulating)
See : http://www.blogofthebiker.com/2016/09/bare-minimum-brazed-frame-gsxr-racer.html
More on topic, a Japenese custom shops version of a BSA A7 Plunger bike using a lot of Brazing
See: http://sgrallying.blogspot.com/2014/06/cyclops-bsa-a7-plate-armor-bobber.html
*NOTE* I find it amusing that certain words are auto-edited on here,, funny that! And on certain forums when discussing Machine shop P R O N, Crankshaft P R O N, or Welding P R O N we spell it that way to avoid spam filters and email issues.. So much for "Taking the P**"