The 'friend' did one of two things, most likely.
1: Connected it to an antenna with a bad coax, bad ground or other fault causing a high SWR.
2: Used a radio that was way too big, and pumped several times the allowable drive wattage into it.
We have found that when there are no penalties for destroying an amplifier with misuse that people tend to misuse amplifiers.
I'll guess that the guy who borrowed it has not suggested paying for the repairs.
The smoked combiner tells you that after one of the two pairs of RF transistors had failed, he kept transmitting until all the smoke was released.
Ask him if he had to open both the driver and passenger-side windows to exhaust the smoke.
Considering the age of this amplifier I would probably classify it as "totaled". Might very well cost less to replace it than to repair it properly.
73