Either the team member is malicious/unethical (but most people aren't), or he's simply not feeling connected to the team and thus does not care.
It's also possible that the team member believes it's a sign of weakness to ask for advice from his team lead (e.g. cultural/environmental issue. I know bosses who think this way too).
Bottom line: the team lead needs to focus on finding ways to promote team cohesiveness. This means leading the team towards working together, through the way tasks are divided and through setting members' personal expectations of each other.
Short term, as a corrective measure, I would have a conversation with the team member, but I advise strongly against personal arguments of any kind. The basis of the conversation should always be professional, always on point and never, ever use platitudes:
"I notice you're eager to connect with
people outside our team, which is good --
they can be a good resource for a fresh
perspective.
I'll try to get you
opportunities to work collaboratively
with other teams more often [but only say
this if you mean it].
For projects that are specific to our
team, I'd like us to work closer together
so that we can keep the work internal and
grow stronger as a team".
Leaders need to keep their promises, so I'd start working closer with the team member by giving him more attention on a day to day basis. I would make sure this is done openly and never behind closed doors (e.g. don't hide it), and never, ever in a condescending or even managerial manner -- think of it as a partnership. Team members need to feel like the lead's number one job is to support their work, not to pontificate from a throne.
Long term, positive reinforcement is key. I would ensure the team member does not receive praise for going against my expectations (remember, I set them clearly when I said "work together, keep the work internal"), and I would visibly praise members working closer together.
I advise strongly against using power of authority to curtail the member's actions in any way -- this means I would never call him into my office and tell him I disapprove of him reaching out to other teams, and I would not hand him simpler tasks as punishment (or as a way of getting him to stop going to other people for help -- that's the same thing as punishment in his eyes).
Managerial authority is a lethal weapon that you just shouldn't draw unless you're ready to kill.
After talking with the team member, working more closely with the team on a daily basis (and taking initiative to more often approach the team member in question if necessary), and stepping up the positive reinforcement, I would observe for a few weeks and see if things improve.
I would use the rest of the team's response as a comparison point to the team member in question, to calibrate my success. Because my actions are public, the entire team will respond through the way they act, how they sound when talking to me, when they don't think I'm listening, etc.
Hopefully this helps!