What do you mean? :)
Sure you can look into your log to get the call stack of this bug)
The bug is in oxwall 1.8 and can be replicated on your demo

We cannot reproduce that on our demo nor test websites. 

If you get the 500 error, follow the instructions I sent you above

Just try to add and remove friend
Parse error: syntax error, unexpected 'public' (T_PUBLIC) in /ow_plugins/friends/controllers/action.php

On you demo login as demo in 1 browser and as admin in another browser.
Open http://demo.oxwall.org/user/admin and http://demo.oxwall.org/user/Demo
And play with "Add to friends" / "Remove from friends"
I can easyly replicate it in different ways.
For example add to friend on 1 page. Then click "add to friend" on the second page.

Provide us with the exact steps, one by one, as we cannot reproduce that on our demo. 

See screenshots

Open http://demo.oxwall.org/ in FF. Login as admin.
Open http://demo.oxwall.org/ in Chrome. Login as demo.

In FF open http://demo.oxwall.org/user/Demo click add to friends.
In Chrome open http://demo.oxwall.org/user/admin click add to friends. Get http://c2n.me/3o8DNRv

Still cannot reproduce that, we get this error message on the second user if requests are sent simultaneously or with a little delay

See screenshot

I can easy replicate it on your server.

Just open page of mutual friend and click Remove from friends.

After the action is performed (add or remove friendship) the page is redirected to incorrect page.

Ok, I was able to reproduce the issue, however it is a close-to-zero situation, nevertheless I'll report it to our devs.