My partner bought shares in a small business in 2011. She recently learned the corporation was dissolved a few years ago. Is it still possible to claim the loss?

You might be allowed to report a loss as ABIL ( allowable business investment loss) 

check out if it fits your situation.

You might have to refile you taxes for the year corporation was dissolved to add the loss there.  

https://www.taxtips.ca/smallbusiness/businessinvestmentloss.htm

https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-return/completing-a-tax-return/deductions-credits-expenses/lines-217-228-business-investment-loss/lines-217-228-what-a-business-investment-loss.html