If you have discovered someone who is or might be frauding the EI program, you may report it to EI office by either calling the 1-800-206-7218 or you may visit your local Service Canada centre. Once the information has been received, Service Canada integrity office will take charge in investigating the claimant in subject.

You may try to contact Service Canada e-Form service to notify.

Source: https://www.canada.ca/en/employment-social-development/programs/ei/ei-list/reports/fraud-serious.html#h2.6