3D Fibs can do the job. In the
log folder, eg.
C:\Program Files\3D-Fibs\log each watch session is stored as a log. If it's a complete match then hopefully 3D Fibs will be able to convert it into a .mat file. It does that automatically for the matches that you play but not for watched matches.
On the
File menu there's an option
Convert log -> JF.mat files. Click on that and it'll invite you to choose a log file. It'll then do what it can with the match and write the best .mat that it can. That may well mean that it writes garbage. I've got about 50 watch logs but most of them start midway through a bot game while I'm waiting for my auto-invite. They produce a mess. I haven't tried watching someone first and then converting the log file after but it might work very nicely.
But I'd recommend that you
get the DelFibs client. It's a lot prettier to watch a game with, bigger board as well. It writes straight to .mat. I've got as many watched matches in that as logs in 3D Fibs and most of them are clean and GnuBGable

.
wow! What an excellent client PB, I'm so happy that I've such a nice friend

I downloaded the program and it's working like a charm. Lots of featues: rating curve, tourneybot integration, toolbar commands, statistics on your various kinds of matches (very very useful), excellent player list info, .....
I really don't know why it's not as popular as javafibs and 3dfibs, or may be that's me who's been so ignorant!
Again many thanks for this helpful suggestion bunny

Regards