Load plugin with PHP

This seems like it would be a standard question but cannot find anything solid in the searches. Sorry if it’s a repost.

How can I load a plugin using PHP within one of my theme files. For a clients site I would rather have a specific plugin load always and not risk them removing it while they edit the others. I don’t mean copying all the plugin files into my theme, just being able to load it if it is active.

I take it this could also be applied to Multisite, where I can just edit my theme with code to load plugins (hardcode it in) rather then logging into every sub-site and placing it within the widgets area.