This may not carry precisely the restrictions on meaning that you are after, but it is an umbrella idea over what you seek:
When it rains it pours
This means that things (good or bad) will happen all at once, once they start happening, perhaps to an overwhelming degree. Your example of the airline is a perfect use case for this expression.
Other possible use cases:
I got a raise at work, and found a twenty dollar bill. When it rains it pours.
I complained about my neighbor's rotten, dangerous shed. He tore it down, which would be good except that now the rats have moved into my shed. The crows have also started a rookery in my lilac tree, so my yard is full of unwelcome visitors. When it rains it pours.
For a long time, I have been unable to find anyone to help me walk my dogs during the day. I promised three different people I would hire them if they changed their schedules around. Now they all showed up at the same time to walk the dogs. When it rains it pours.
This expression is a fairly new phrasing of an older expression, coined by the Morton Salt Company to publicize the marketing point that when the weather is humid (When it rains...), their salt still flows freely (it pours). http://www.mortonsalt.com/our-history/history-of-the-morton-salt-girl
The original, it never rains but it pours would work also, but is much less common and people might actually try to parse it, instead of just "knowing" the meaning.
Another option -- depending on if you will be replying in email, in detail -- is to thank her happily for the information in the email, respond to that as appropriate, and just not mention anything about the apology one way or the other. In a face-to-face conversation, you could say, "It's all right" to an apology or some other small-talk, but making a point to address the "sorry I'm late" in an email would generally need to be done with humor or other truthful understanding about busy lives. E.g., after an ice storm, I was able to write things like, "No worries; turned out I lost Internet for three days anyway!"
(But if you do acknowledge the apology instead of just letting it pass off and indicating your no-hard-feelings by how you react in the rest of the message, another possible reply would be, "I understand; life gets busy. Thanks for getting this to me!")
Best Answer
"The blind leading the blind."
From Wikipedia: The blind leading the blind is used to describe a situation where a person who knows nothing is getting advice and help from another person who knows almost nothing.
Example: "Alice just bought her first car and is asking Bob for driving tips. I don't know why, since Bob is infamous for how often he gets caught violating traffic laws. It's the blind leading the blind."