Look, if she keeps asking you for advice, then she is making it your business, isn't she? It's not as if you've waded in with unsolicited advice. I often wonder about the quality of some of the friendships posters on here have, because we all need friends who will tell us how they see our behaviour and actions, especially if it is harming ourselves and others. As long as we are sure that our friends love us and that their advice is from a good place, good friendships withstand honesty.
If you are determined to help her, then you might want to try my approach of "If you really want my advice, then I'm happy to give it, but I will be very challenging and you might not want to hear it." This is what I said to my friend as described downthread and it was therefore her choice to come and talk to me. We have been friends for over 30 years, so she knew I had her best interests at heart, loved her and would go on loving her.
It might help though if you challenge some of your own thinking. She's not acting like a man, because women are just as capable of having an affair despite a good marriage. She is at least being honest that there aren't any problems in her marriage and she's not vilifying her husband. She's also being honest when she says that the only problem is that the "spark" has reduced over the years. It is an absolute myth that women's marriages need to be bad, for them to have an affair. I suspect what's happened here is a massive over-reaction to a crush.
However, where I think your friend might be falling into a gender trap is in how she describes her feelings. I have often noticed that when some women have affairs, they cannot acknowledge that all they are after is some no-strings sex, so they frame their feelings as "love" when what they are feeling is "lust". Because they convince themselves that they are in love, this leads to some very poor decision-making.
A pragmatic, wiser friend can be a great help here in analysing her true feelings for this man. It goes without saying too, that if the only problem in her marriage is that it lacks spark, having an affair will cause what spark there is, to be extinguished completely.
She is evidently too far gone here if she is saying that if her H left, she wouldn't mind. The truth, I suspect, would be rather different. It leads me to wonder whether she is therefore being entirely truthful with you and that she has had sex with the OM.
One of the best ways of helping her will be to get her to take responsibility and project the future. People immersed in the insanity of an affair are notoriously poor at working out the true consequences. Some of the worst for her are that she might lose the right to be the resident parent and at the very least, she will be looking at shared parenting with her H, who doesn't deserve to lose living with his children.
Statistically at least, it is highly unlikely that the new relationship with a 20 year old is going to withstand a bitter divorce, hurt children and the lure of women his own age.
The price she is likely to pay for what undoubtedly was an over-reaction to a perfectly normal crush, could be enormous.
Now I well understand that as her friend, you are horrified that this could become her fate and so since she has involved you against your will, the most valuable thing you can do for her is to be the pragmatic, wise ally. It would be a sign of a shallow friendship if you sat on the sidelines saying nothing, while this car crash happened.
Now, bear in mind that she will have rather shallower people in her life who might collude with her and urge her to find grievances in her marriage that simply don't exist, so it would be fair to warn her of this too. This is partly because some women simply cannot get their head around a woman having an affair for reasons other than marital unhappiness. What happens then is that the denial and romantic idealism sets in even firmer and terrible decisions are made, all in the name of "love".