My girlfriend had applied a standard visitor visa last year and her stated reason was tourism. Is it OK for her to visit me now, as it's now for a different reason?
With a Standard Visitor Visa, she can come and go and do any activity listed in Appendix 3 of the Immigration Rules Appendix V visitor rules. This holds true even if the stated purpose on the application now differs from the current purpose. Among them, you'll see
Tourism and leisure
3 A visitor may visit friends and family and / or come to the UK for a holiday.
Of particular relevance, as this question is frequently asked on this site, @GayotFow explained that:
On 24 April 2015 all existing visitor visas, including family visitor, business visitor, student visitor, child visitor, and others were automatically converted to the 'standard visitor visa'.
The controlling link if you are interested is the Immigration Act 2014.
For additional reference: Can I travel to the UK for business on a C visit visa?