Can one apply for a Schengen visa while visiting the UK?

No, she has to apply from Pakistan.

Visa applications for UK residents are to be submitted at the Swiss Visa Application Centres in London, Manchester, Cardiff or Edinburgh (UK).

You mom is not a UK resident but rather a visitor. Therefore she has to apply from Pakistan where is a resident.

There are very rare exceptions when applications are entertained from outside your home domicile - but these generally have to do with emergencies.

You can apply as early as 90 days before your travel date.

To apply from Pakistan she has to apply via VFS/Gerry's which have offices in Islamabad, Karachi and Lahore (no appointment required); or she can apply directly via the embassy but you have to schedule an appointment in advance, and these are limited.

Documents required:

• Photographs
• Biometric data (fingerprints and photos) collected in person at VFS or the embassy.
• Visa application
• Fees (including service fees if applied via VFS)
• Passport (valid for 6 months after travel date)
• Proof of funds (bank statements, etc.)
• Travel itinerary (travel can be Pakistan - London - Switzerland, it doesn't have to be direct to Switzerland)
• Evidence of home ties
• Hotel accommodations in Switzerland

For your mom as she has a long term visa for the UK, I believe her application will have a smoother process of getting approved.