I am so sorry that this has happened to you. From now on you have to put you and your DC first. Ask him to leave, he has made his decision and therefore needs to move out.
One thing at a time, but if you get WTC you need to ring them and start a single claim. If you rent you may get help with housing benefit. You can get your council tax reduced by 25%. You may be able to get your payments spread over 12 months instead if 10 too, some councils are doing that now, mine does.
Make sure you have details if all your finances including his pensions, life assurance, savings etc.
On Monday, make an appointment to see a solicitor and find out what you may be entitled to etc and if you know his salary you can go online and calculate CSA payments.
All these things will empower you go know that you can survive financially and that will be one thing off your mind.
You can feign illness today and then be honest with your friend when she gets back from honeymoon.
If you can find any proof of adultery, emails, Facebook etc then print it off so you can use it to divorce for adultery.
Look after yourself, try and eat something, just little and often.
Just remember that it is about you and your DC from now on. He will turn into somebody that you no longer recognise so be prepared for that.