Retired Mom Likes the Computer

Whenever I hear of someone who spends way too much time doing anything, the first question I ask is what does it hurt? Certainly spending most of your time doing the same thing limits your opportunities and conversation topics, but there is no healthy limit based on age. Each situation must be considered individually based on consequences.
The serious consequences that can occur have to do with maintaining a job and supportive relationships. Your mom is retired, so she has a lot of free time on her hands. Does the computer keep her from taking care of her home, her bills, or other obligations? Does she maintain her relationships with friends and family so she can have some support if she needs it? If there are concrete consequences to her limited recreation choices, that may be helpful for persuading her to broaden her interests.
Sometimes people will isolate and limit activities when they are depressed, or anxious and/or irritable around other people. This might indicate some underlying issue that may not be so obvious to others. Another pitfall to watch is how she is spending her time on the computer. Is she spending lots of money gambling or compulsively buying things she can't really use? Is the amount of money she's spending so excessive as to limit her housing or her health services? If she has lost her ability to make good choices about her finances or health, you may need to talk to a lawyer about conservatorship or even guardianship.
If the consequences of her actions seem limited to nothing more important than what she can talk about as "news", then perhaps she's making a choice that she has the freedom to make.