To get a the path to a plugin you can use
plugin_dir_path(__FILE__) - but this obviously has to be called from within the plug-in.
How can you reliably get the path to plugin B (
pluginb/pluginb.php) from within plug-in A?
Edit: Its assumed you know the slug of the plug-in you're after (actually you can get them from
get_plugins()). But I would like it to work generally.