Let them know that you're there for them. Try not to ask in a passive way, like saying "let me know if you need help", although the sentiment is nice and you mean it, the chances are your friend might still not reach out even after being told he's more than welcome to. Instead, try asking active questions like "do you need anything?" or "what can I do to help". Your friend is more likely to give a clear answer this way about what they actually need. And that way you are making the first step and not putting that pressure onto them. And don't be afraid to ask the questions again at a later time.
Like brink said though, your friend will probably need time to process and grieve on their own, so give them space if they need it. But a quick text asking if they need anything never hurts.