Provided their baptism was Trinitarian (in the name of the Father, the Son and the Holy Spirit), their baptism is generally recognised by the church.
However there is a bit more to it.
Normally they spend a bit of time going through RCIA (Rite of Christian Initiation of Adults).
There is a link here which might give you some explanation of the RCIA process and who needs to do it. One of the reasons baptised Protestants do RCIA is that the beliefs they have brought with them may not necessarily agree with some Catholic teachings eg. the Eucharist, Papacy, Sacraments, Marianism etc. They need to be clear what about what they are getting into.
“Jesus spoke to them again, saying, ‘I am the light of the world. Whoever follows me will not walk in...
ololcc.org
I'm a former Protestant and I went through RCIA myself, quite some time ago now. I attended the sessions at the home of another couple whom I've gotten to know well since then. Not so long ago they said to me they didn't think I'd go through with it as I asked so many questions. Then they said "But you're the one who has stuck around and done things."
To really find out what is required, assuming you are genuinely interested and not just asking out of curiousity, I'd suggest you talk to a priest.