My 2p's worth, having been through all this **** in the past ...
Both myself and my sister found out our mother was seeing another guy independently, and for some reason didn't tell each other about it. My sister saw a text on her phone when it beeped in her handbag, and I checked her email account (I used to cycle through all the family's hotmail accounts from time to time to keep them alive). We only found out each other knew already when she properly announced (by way of buying a house with him) that she was off after 23 years of marriage. It wasn't great to feel partly responsible, either, as it was through me that they met again - he was an old friend / ex of hers that I went to do some work experience with. This was all during my A2s, which didn't do much for my motivation at the time.
So, at the time, my Dad was torn apart and suffered quite seriously with depression. Both me and my sister weren't about, as I'd just gone to uni, and I can tell you it's not nice speaking to your Dad on the phone and knowing that he's quite seriously contemplating suicide. When I was back in the holidays, I came home twice to find notes inside the door saying that he wasn't coming back. Luckily for all of us, he never went through with it.
I suppose the flip side is that time is a healer. Both my parents are now remarried (my Dad was 58 at the time). My Dad's new wife is lovely, as is my Mum's new husband. They're much happier now independently than they were together. At the time, I wasn't too keen on the situation, to put it mildly, but I owe my parents everything in terms of my education and chances in life, so who am I to say how they should be living their lives? All people should have the right to be happy, at the end of the day.