Does anyone know the deal with applying for a rental with someone inspecting on your behalf?

I always thought that it was fine to have someone inspect on your behalf, but a friend recently encountered an agent who wouldn't allow it. Does anyone know if that's universal, or a common policy with REA's, or if it's unusual? Already checked TUV and found nothing mentioning it on the site.

Thanks!

Edit: the agent for the house I want most will accept an application if my mum inspects for me! Still no idea if that's common practice or not but hopefully I don't have to find out