Jeff, are you a window cleaner or do you know something about fabric cleaning? this will effect the answers available,
if you know nothing about fabric cleaning then you need to realise that marker pen isn't usually soluble with a water based cleaning agent ( vanish, 1001.... etc) so it will need to be removed with a solvent solution. Asda sell a solvent cleaning solution, its in the laundry section. you could try this, follow the instructions on the bottle.
or does your wife have some Acetone nail varnish remover ( the liquid not the gel) I believe this could work on some pen marks, but you need to work carefully using cotton buds
perhaps some one could comment more on the use of acetone to remove stains as my knowledge is a little limited in this area
Mike