Best to ask what glue it actually is, if they know. Have they had any new flooring fitting? Could be splatter from that.
Type 20 solvent up call cleaner could do the job, hot hot water. Use plastic to scrape it off if possible. With GREAT risk you can use pipe freezing spray to make the compound brital which of course in turn could damage the upvc. I've cubes or ice packs could work but less effective shoukd the 'freeze off method work at all. The glue/ adhesive could have bonded with the upvc almost even melted in to it as such.
The customer would have to understand and agree it is a trial and error job that could leave the door less than perfect! I would get an agreement in email/writing
You say it's a blue door,. Some upvc products are sprayed the colour rather than the plastic being a blue colour itself, if the case is a painted upvc it could mean a repaint.