1 user having trouble logging in

I have one user who cannot login to her site. I have logged in with her credentials from other computers and phones in different locations and I have no problem.

The user is on a mac, running mavericks and has tested with firewall switched off, has tried Opera and Chrome, but when she tries to login she gets a NO DATA RECEIVED message (Err_Empty_Response), ‘Please reload webpage’, and then nothing happens.

I have deleted and reloaded the wp-login file. She managed to log in for a short time twice before the error message came up, but one of those times she saw the dashboard without any style sheet. And one of those time I tried deactivating the plugins.

I asked her to login to one of my sites on a different server and she had no problem.

Please help, the user needs to maintain the site but can’t even get in.

Thanks!