My go to was denatured alcohol, melts away the rotten foam and used sparingly to clean a spot on a headliner fabric that ended up with it on it. I used a spray bottle to apply it to the stripped headliner board and also to clean surface where the sunroof slider slides into. Protect the interior in case of a drip.

Do the right thing and replace the top, the heat seal failure is just the obvious and noticeable wear and tear. The rest of the top is worn out . Please do not waste your time and your customers money trying to patch this. Replace it with a quality top , stand up to do the right thing , worked for me for over 30 years.

I use the orbital sander method on foam that is not quite rotted out, wire brush on the harder headliner boards. My magic way to get those Import headliners super clean in denatured alcohol, put it in a spray bottle , after scraping the best you can with a hand wire brush saturate a couple square feet at a time and wipe clean with an old towel cut up into rags. Also use it to clean up the sunroof tray area. By spraying in and use an old scrap of foam headliner rolled up and taped to metal yard stick.
Be sure to cover all the seats in case of a drip. I use it to get any foam residue that had turned to a greasy type mess. Good luck!

Just say no , they never fit . You will only be frustrated. Tell your customer if he wants it done correctly to pull the windows. These abs headliners were never designed to fit the existing trim. Especially with s customer trying to save money? It will be a loser.

The problem is the price point , try selling a $800 plus carpet for a 10 year old car. I agree the ACC carpets do not fit well especially with the jute glued to the back . I price the jobs so high that I hope they pass on the job . To much for an average result. I just did an install on a 1972 Corvette, ACC made the carpet and the fit was terrible. Luckily the customer brought me the carpet and I charged
According for the time I spent. I always price the factory carpets because the labor can reduce the overall cost and the customer will be much happier.

I have used denatured alcohol in a spray bottle.
It is the only thing that I have tried that dissolves the foam and leaves a clean surface. Sometimes you need to use a razor blade to clean off a hard surface like a sun shade from a Mercedes when doing a headliner replacement. Also PPG DX 106
Surface prep works great for cleaning the orange residue from fabric without damaging the cloth.
It is quite expensive compared to denatured alcohol but it is different.

I have used a Consew 226R for years , best thing I did was upgrade it to a servo drive,
so much smoother operating . Worth every penny

Seat I just did for a customer who had this material already in the door panels etc. Changing the Bucket seats to a bench seat.