No, don't use wallpaper paste to put up polystyrene wall covering - it's water based and not strong enough.
Decorlite have a ready-prepared paste (B&Q etc) specially designed for the purpose - more expensive but well worth it. Your friend then needs to apply vinyl wallpaper to cover the polystyrene (using the same paste) NOT wall PAPER, which shrinks when drying and could damage the polystyrene.
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