Anytime I consider a makeshift repair I consider 2 things; likelihood and consequences of a failure. In fact those are critical issues engineers deal with all the time. For example it's why aircraft have redundant systems and cars don't.

In this situation let's deal with potential consequences first. Of all the places to have a sudden catastrophic failure, seat stays would be among the least likely to lead to a serious injury. So, I'd be far more willing to try my luck there than, say, the downtube.

As to likelihood, that depends on how damaged they are now. If the damage is more like bruising like from a side impact, odds are that they just need a splint to prevent buckling. Therefore filling with structural foam may be very workable.

The OP also factor his options. Here in the USA there's easy access to qualified help, so it's only a question of cost. However, that may not be true in the Philipines.