In this counseling answer:
•Without full information from the husband as well we must be careful of how we advise her.
•Seek a third party to intervene between the two; an imam or marriage counselor.
•Read daily supplications and make supplication to Allah for His protection from magic.
•Support from friends and family is essential at such times.
•Take care of yourself: do things you enjoy and be with other people.
Wa alaikum salaam wa rahmatulahi wa barakatuh sister,
May Allah reward you for looking out for a sister who has been experiencing difficulties in her marriage.
It must be a very traumatic time for her right now as she feels abandoned and is left alone to raise their child. It is disturbing that a practicing husband could behave in a way that is seemingly inconsistent with Islamic values.
However, without full information from the husband as well we must be careful of how we advise her. Perhaps there was a reason why the husband left that your sister may or may not be aware of, but such factors would be very important to take into consideration to give the most appropriate advice.
Seek Help from a Third Party
Given that the full details of the situation are with her, and is between her and her husband, she might be advised to seek a third party to intervene between the two. She needs a person of knowledge such as an imam who will be able to listen to both sides of the story and advise according to Islam.
This will give them both a chance to have a say in what is going on and express their feelings to each other in the presence of a mutual party who is impartial in the relationship and therefore has no emotional attachment to either.
This would also be best for their daughter than any potential arguments that might erupt otherwise, especially when there are accusations of the involvement of magic which can often heighten emotions further.
Protect Yourself from Magic
Regarding the accusation of magic use, this is something to very careful with as it is a serious accusation to make and glean lead to major arguments and family breakdown. Instead, it is best to focus on protecting oneself from magic by reading daily supplications and make supplication to Allah for His protection.
For you as an outsider, even though it is best not to advise or get involved in their marital situation, you can help by being a support to her during this difficult time. Right now, she will be feeling alone and abandoned and will likely need support even if it is just to know that someone is there for her that cares.
Make sure to check in on her regularly just see how she is doing. Offer to help out where possible, for example now that she is alone with her daughter, if she needs help with childcare perhaps you could volunteer to take care of her daughter whilst she takes some time to herself.
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It is likely she feels quite lonely also so you might go and spend some time with her or encourage her to do things she enjoys and be with other people.
This will help her to feel better about herself at a time where she is probably feeling quite bad about herself since her husband left her for another woman. This will put her in a more suitable and positive frame of mind to deal with her situation more effectively.
May Allah see her successfully through this distressing situation and reward your efforts to assist her.
Disclaimer: The conceptualization and recommendations stated in this response are very general and purely based on the limited information provided in the question. In no event shall AboutIslam, its counselors or employees be held liable for any damages that may arise from your decision in the use of our services.