My cousin has some malware on her computer, she's not very tech savvy and lives far away. Can I just connect to her computer with team viewer and just use a rescue tool?
Yes she can. I do that from time to time. I usually run scans with Zemana, AdwCleaner, Malwarebytes and occasionally HitmanPro too. If they are using an expired an antivirus, I will uninstall it and install a replacement.
It depends on the infection, sometimes it's not always that simple... She could always join the forum and make a thread over in the Malware Removal Assistance area, where she can provide the system logs and get some custom scripts to clean her system by a malware removal expert on the forum: Malware Removal Assistance
That would be more reliable than you connecting and just running a couple scans.
In a very heavily infected computer it may not be an option. But, I have cleaned plenty of computers that way, and it's basically the same as what I do when I am working directly on a customers computer or if one of my computers becomes infected. It's just makes things easier being able to to do it locally instead of over TeamViewer.