You can't make the oil modifiable again, no.
If your friend added the drying linseed oil and turpentine to existing oil paints (and not just pigments), it really depends on the ratio whether it improved drying time or not.
We have several answered questions on the drying time of oils, which I recommend perusing, or sending to your friend. You can find these on the right side of this page (they were automatically linked to your question based on keywords and tags).
The general response to how long the drying of oil paints take is that it depends on so many variables that we can't really give any concrete answer: humidity, temperature, the aforementioned ratio of oils and pigments and other additives, the amounts and types of oils and pigments used, &c. all have a bearing on drying time.
Depending on the thickness and current state, your friend might be able to add additional layers, but be sure to heed the fat over lean rule: any new layer must consist of more oil than the former, so the painting will dry from the bottom (canvas/panel) upwards, and provide a (more) stable whole.
I realize you mention your friend "doesn’t want to just paint over the existing one", but he has to retain the transparency anyway (because of the required addition of oil), and this is the only stable way to adjust the current painting.