She's been negligent about staying in touch with friends.
Tips
grammar
Pattern: 疏于 + verb/noun (一个事项), "to be remiss with respect to X." Always carries a faintly accusatory/self-critical tone. Common collocations: 疏于管理, 疏于防范, 疏于照顾, 疏于联系. Never positive — you can't 疏于 something good.