My friend is invited to my private server in Roblox, and it says She needs Download Roblox on Computer even though she plays on Mobile
Does she already have the app installed? If she is just on the website, it will not work correctly. She needs to have the app installed and use that instead of the website.
She's on Android/iOS. She must search Roblox from the App Store of the device, install it, and launch it from there and she will be able to join.