She can feed her baby formula or something until it's totally safe to breastfeed again. Or pump some milk beforehand and store it in the fridge.
I can see where she is coming from. Jeez I'm almost at the baby stage myself and can't imagine going so long w/o any mind altering substances. Guys have it so easy in that and many other baby related departments. They shouldn't even be allowed to speak here (j/k).
If she can somehow do it w/o harming the baby in anyway, what the issue then.
New mothers have the right to go out. There are family members that can baby sit. Then wait until the drug is outta the body and feed w/ formula until then. What's the harm then? Both baby and mommy are happy.
Some internet info:
To the best of our knowledge, this issue has never been formally studied, however LSD is metabolized fairly quickly in the body. While any trace amounts will most likely be gone from breast milk within a day (24 hours), to be safe, you might want to give yourself at least 2 days to allow it to pass. You may also want to pump whatever milk was produced during this time and discard it.
About 1/2 life of LSD
This is how half-lives work. Drugs with first-order kinetics (such as LSD) are broken down in a process of exponential metabolism, which usually takes 6 half-lives until the drug is "gone" (6 half-lives of LSD would 15 hours).
Since the half-life is the time it takes for 50% of the drug to be broken down, the next half-life will break down 50% of the remaining 50%.
In other words, taking 100ug of LSD will result in the metabolism of 50ug in the first half-life. The following half-life will metabolize 50% of the remaining 50ug, leaving 25ug behind. After that, the next half-life will metabolize 50% of that, leaving 12.5ug behind and so on and so on until there is a negligible amount left.
She brought life into the world and Im sure tripping after that would be something that I personally would want to do 
If the mother feels that the experience will not harm the child... who are we to judge?
