I wouldn’t do it because there’s no weld on the inside. Strictly speaking a welded aluminium frame is a monocoque because the heat treatment after welding raises the temperature of the crystalline structure to a level where the whole frame becomes a homogeneous mass rather than several parts joined together with weld.
But unless the welds have a backing run on the opposite face, it’s likely that there will be far less material in these areas and they will still be a weak point.
Cannondale used to smooth the welds by building them up much bigger than they needed to be before dressing them smooth and then heat treating. This is why Cannondales used to be so horrendously expensive back in the day because the process was incredibly time consuming.
Thats said, you did say it was your other half’s bike, so crack on!